The present invention relates to the production of ammonium phosphate, and more particularly, the present invention relates to an improved process for the production of relatively pure diammonium orthophosphate (DAP) from impure wet-process phosphoric acid (WPA). It is commercially desirable to utilize such wet-process phosphoric acid rather than the purer electric furnace acid in the production of feed-grade, solution fertilizer-grade, and technical-grade diammonium phosphates. These upgraded phosphates are established commercial chemicals and are used in animal feed supplements, liquid fertilizers, detergents, and other uses.
Generally, wet-process phosphoric acid is exceedingly impure for such uses. It is ordinarily prepared by acidulation of phosphate rock and contains relatively large amounts of acid-soluble impurities, such as sulfates, fluorides, and fluosilicates, salts of aluminum, magnesium, iron, calcium, and other metals as well as dissolved and suspended organic matter. These may vary, for example, in filter-grade acid from 2.5 to 3.0 percent SO.sub.3, 2 to 3 percent fluorine, 1.25 to 1.75 percent Fe.sub.2 O.sub.3, 1.25 to 1.6 percent alumina, and 0.25 to 0.4 percent CaO, though the quantities set forth above are only illustrative and are quite variable.
Ammoniation of the impure wet-process acid to form diammonium phosphate causes most of the metallic and fluorine-containing impurities to precipitate in the form of gels and/or very finely dispersed solids. These gels or finely dispersed solids settle very slowly and blind filter media, which makes their separation from the solution too difficult to be practical on an industrial scale. Thus, when diammonium phosphate is crystallized from the solution, the slurry contains the precipitated impurities. Because of the above described properties of the precipitated impurities, it is impractical to separate them from the diammonium phosphate crystals by selective filtration or other means and, in addition, they make separation of the diammonium phosphate from the mother liquor by filtration or centrifugation exceedingly difficult.
Various methods for purifying wet-process acid have been tried. These include ion exchange, dialysis, selective crystallization, solvent extraction, and solvent precipitation. For economic or technical reasons, none of these methods have gained extensive industrial acceptance.
One recent breakthrough in overcoming the disadvantages of wet-process phosphoric acid is taught and described by Sheridan in U.S. Pat. No. 3,920,796, Nov. 18, 1975, the teachings of which are incorporated herein. Sheridan teaches that melamine is added to impure wet-process phosphoric acid to form crystals of relatively pure melamine orthophosphate, C.sub.3 N.sub.3 (NH.sub.2).sub.3.H.sub.3 PO.sub.4, which are separated from the mother liquor and caused to react with an aqueous solution of ammonia, thereby forming crystals of melamine for recycle and a solution of purified diammonium phosphate. As may be seen from said teachings of Sheridan, specifically but not exclusively FIG. 1 thereof and the attendant descriptive material in the specification, the bulk of impurities which are usually a problem associated with the wet-process phosphoric acid are separated from the melamine phosphate cake via the filtrate from Sheridan's filter 2. The melamine process as taught by Sheridan, supra, provides an improved process for the production of ammonium phosphates; however, it does not produce solid, crystalline diammonium phosphate without the use of expensive, energy-consuming evaporation or other dewatering procedures.